One of the few commonalities that lone wolf Islamist terrorists share—whether in San Bernardino or now Philadelphia—is their Sunni Muslim background. They are never Shia.

Of course, there is Hezbollah, the Shia terrorist group in Lebanon, which has a long list of bombings and kidnappings attributed to it. In 1992, it attacked the Israeli Embassy in Buenos Aires; in 1994 it bombed an Israeli community center in the same city; and in 1996, it blew up the Khobar Towers in Saudi Arabia. But the difference is that these were coordinated attacks. There are also other Shia militias in the Middle East, such as the Mahdi Army in Iraq, and Amal in Lebanon. But none of these Shia groups are associated with inspiring individuals to commit violence on their behalf. Almost without exception, Shia militant groups pursue a direct geopolitical goal, such as pushing the United States out of Baghdad or the Israelis out of Lebanon, and are under the direct control of Iran or Hezbollah.

This is partly because there are far fewer Shias than Sunnis. Shias make up about ten percent of the world’s Muslim population. Even if the rate of lone-wolf terrorism were the same in both groups, the number of Shia terrorists would be exponentially smaller than the already low Sunni number.

The shape of each sect might also make a difference. Sunnis do not have as clear a clerical hierarchy as Shias do; as a result, there are more Sunni interpretations of Islam, and thus, more potential for deviation from the mean. There is no Sunni equivalent to Iraq’s Grand Ayatollah Ali al-Sistani, a widely recognized supreme authority who advocates for moderation.

But the real key to understanding the current state of terrorism lies in the year 1979, which is marked by three seminal events. The first two became symbols of the power of political Islam: the Iranian Revolution and the start of the Afghan war against the Soviets. The eventual successes of both were seen as